Tooth and hair are formed by invagination of epithelium. Invaginated epithelium interacts with neural crest derived mesenchymal tissues, and then formed tooth and hair. However, mechanism of organ determination has never clearly understood. Sox21 is one of the transcription factor, and expressed in dental epithelium. Sox21 knockout mice showed hair formation in incisor indicating that Sox21 may be involved in the determination of organs. Decrease Sox21 inhibited ameloblast marker gene expression, and induced hair marker genes including keratins. ChipSeq analysis showed that Sox21 directly binds to Anapc10 gene promoter region and regulated their expression in dental epithelium. In Sox21 knockout mice, Anapc10 expression was decreased in tooth germ. Further, Sox21 over expression induced Anapc10 expression. These results suggested that Sox21 regulates Anapc10 expression and organ determination.